Muscovy duck retinoic acid-induced gene I (MdRIG-I) functions in innate immunity against H9N2 avian influenza viruses (AIV) infections

Retinoic acid inducible gene I (RIG-I) is a cytosolic pattern recognition receptor that senses pathogen-associated molecular patterns (PAMPs). Muscovy duck (Cairina moschata) is a large duck different from other species of ducks, and is more susceptible to some microbial pathogens. In this study, the Muscovy duck RIG-I gene (MdRIG-I) was identified. Quantitative RT-PCR showed that MdRIG-I mRNA was widely expressed in different tissues, especially in those with mucosa. RIG-I null DF-1 cells transfected with DNA constructs encoding MdRIG-I or CARDs domain can activate IRF-3 and NF-κB to up-regulated activity of IFN-β promoter. The components of the signaling pathway downstream of RIG-I in mammalian cells including IRF-3, NF-κB, IFN-β and the IFN-stimulated genes Mx-1, PKR and MDA5 were significantly up-regulated in CARDs-overexpressing-DF-1 cells. Implicating RIG-I in the antiviral response to an infection in vivo, we found that RIG-I expression in brain, spleen, lung and bursa were up-regulated in ducks challenged with H9N2 avian influenza virus (AIV), whose six internal genes were closely related to the H7N9 and H10N8 AIV. In vitro, DF-1 cells transfected with MdRIG-I plasmid can respond significantly to H9N2 AIV, evident through enhancement of IFN-β promoter activity and decreased virus titer. Altogether, these results indicated that MdRIG-I is a novel member of RLR gene family, engaging in the early stage of antiviral innate immunity.